Considerable effort has been made in recent years to produce various types of ceramic structures for use in high temperature applications such as gas turbine engines, rocket nozzles, and turbo chargers. The relative light weight of ceramic structures as compared to metal alloy structures provides significant improvement in many cases. Such improvement is particularly notable in the case of moving parts, since the inertia of the structure is substantially less when comprised of ceramic materials. Also, such materials typically have a high modulus of elasticity and a low thermal coefficient of expansion, which are desirable characteristics in high temperature dynamic applications.
One of the ceramic materials which has been investigated for high temperature structural applications is silicon nitride. Typically, silicon nitride bodies requiring close dimensional tolerance are fabricated from silicon nitride powder. However, in the manufacture of silicon nitride bodies the use of bonding agents, sintering aids, or reaction sintering techniques has typically been necessary. An example of the use of such techniques is disclosed in Washburn, U.S. Pat. No. 3,193,399, in which granular silicon carbide is mixed with finely divided silicon and a finely divided cyanamide compound and the mixture is fired in an oxidizing atmosphere. The material resulting from this technique comprises silicon carbide bonded with silicon nitride and what is probably silicon oxynitride. The ability to manufacture parts very close to the desired final shape at low cost using these techniques is a significant advantage.
A disadvantage of bonded and sintered silicon nitride is that it is subject to oxidation attack. This can cause deterioration of the surface of a silicon nitride body, particularly at high temperatures. Moreover, room temperature strength may be lower than desired. For this reason, attempts have been made to provide outer oxidation resistant coatings on bodies of particulate silicon nitride and silicon carbide are good high temperature oxidation resistant materials which would make desirable outer coatings. For example, silicon nitride has been used to coat materials other than silicon nitride; see, for example. U.S. Pat. Nos. 4,090,851 and 4,356,152. Similarly, silicon carbide has been used to coat materials other than silicon nitride; see, for example, the report of Oak Ridge National Laboratories, ORNL/TM-9673, page 104, publication date September, 1985 entitled "Ceramic Technology for Heat Engines Project Semiannual Progress Report for Period October 1984-March 1985". However, useful silicon nitride coatings on particulate silicon nitride bodies have not, prior to this invention, been successfully achieved by conventional vapor deposition techniques. Similar problems exist with silicon carbide coatings on particulate silicon nitride bodies. In both instances, adhesion is very poor resulting in a spalling or flaking off of the coating under minimal stress conditions.
ln addition to providing oxidation resistance, dimensionally additive coatings are provided on partially consolidated, particulate ceramic bodies, such as by chemical or physical vapor deposition, to provide an envelope around and to assist in high temperature isostatic pressing of the body. This pressing initially densifies the body and improves its mechanical properties. Without an encapsulating coating, the high pressure gas used in isostatic pressing infiltrates the porous body and may cause disintegration of the body during the pressing process. Coatings of various materials have been employed to encapsulate partially consolidated, particulate silicon nitride bodies to permit hot isostatic pressing. Unfortunately, if the coating is not useful in the final product (e.g. is not resistant to oxidation at high temperature), it may have to be removed from the body after the isostatic pressing process. If it were possible to coat partially consolidated, particulate silicon nitride bodies with silicon carbide or silicon nitride, such a coating would not only facilitate hot isostatic pressing of the body, but the coating could be left in place after isostatic pressing to form a high temperature oxidation resistant surface for the body.
As previously mentioned, attempts in the prior art to coat particulate silicon nitride bodies with either silicon nitride or silicon carbide using chemical vapor deposition (CVD) or physical vapor deposition (PVD) have been unsuccessful. This is because inadequate adherence of the deposited coating to the surface of the sintered or reaction boded silicon nitride substrate has not been achieved.
In the report of Oak Ridge National Laboratories, supra, at pages 104-105, a proposed method is described for improving the adhesion of physical vapor deposited, sputtered, or plasma sprayed coatings of zirconium oxide materials to the surface of reaction bonded or sintered silicon nitride. This method involves the etching of the as-received and oxidized surfaces of the substrate with aqueous hydrogen fluoride (HF) before applying the outer coating. According to the authors of this report: "The change in surface roughness observed suggests that HF etching should not significantly improve coating adherence through mechanical interlocking of the coating with substrate surface features."